The present invention relates to an optical connector using a polarization-maintaining optical fiber.
Optical fibers with high internal birefringence have been developed to maintain the polarization plane of the transmitted light over a long distance. In the most common type of polarization-maintaining optical fiber, the birefringence is caused by a stress anisotropy in the core. The stress anisotropy is achieved by doping a material of a larger thermal expansion coefficient than that of the silica substrate. As an example of such a polarization-maintaining optical fiber is well known the structure described by Y. Sakaki et al. in "Fabrication of Polarization-Maintaining Optical Fibers with Stress-Induced Birefringence", REVIEW of the Electrical Communication Laboratories, Vol. 32, No. 3, 1984. The optical fiber described in the paper comprises a core and parts arranged around the core and applying stress to the core in a fixed direction. In the optical fiber, by making one of two orthogonal polarization planes of the light coming incident on the optical fiber coincident with the direction of the stress on the core (main stress direction), the polarization plane of the light propagating over the core is maintained in the direction of that stress. However, if some external force prevents from maintaining the polarization plane, the polarization plane will rotate to invite deterioration in the polarization extinction ratio (power ratio of the two orthogonal polarization). Therefore, utmost attention has to be paid to maintaining the polarization plane. However, as no adequate attention is paid to the preservation of the polarization plane in a conventional optical connector using a polarization-maintaining optical fiber, there is the problem of rotation of the polarization plane, resulting in a substantial deterioration of the polarization extinction ratio.
To further elaborate on this point, in a conventional optical connector, when a polarization-maintaining fiber is fixed within a hole of a ferrule made of glass or some other ceramic with adhesive, the fiber is fixed off the center of the hole, resulting in uneven thickness of the adhesive layer. The uneven thickness of the adhesive layer, affecting the fiber as an external stress at the time of hardening of the adhesive, disturbs the direction of the internal stress applied in advance to the fiber. As a consequence, the polarization plane will rotate in response to the external stress, inviting a substantial deterioration in the polarization extinction ratio.
If a polarization-maintaining fiber having a polarization extinction ratio of over 40 dB, for instance, is used in a conventional optical connector, the polarization extinction ratio will deteriorate to somewhere between 15 and 40 dB, resulting in a fluctuation of the deterioration of the polarization extinction ratio from about 0 to 25 dB.